Giant isopods are opportunistic scavengers on a food-poor seabed. Sinking carrion arrives unpredictably, so the animals gorge when it does, distending the gut until movement becomes difficult, then metabolise the meal slowly. Their metabolic rate is low, and captive specimens routinely feed at long intervals.
The Toba Aquarium specimen known as No. 1 took its last recorded meal in January 2009 and refused food thereafter until it died in February 2014, a documented fast of about five years. Aquarium staff continued to offer mackerel throughout. A necropsy did not identify a clear cause of death, and no fully accepted explanation for the fast exists.
The usual explanation offered for the isopod's size is deep-sea gigantism, the tendency for some deep-water invertebrates to grow larger than shallow relatives. The pattern is real but the cause is not established, with cold temperature, high oxygen solubility, delayed maturity and scarce predation all proposed. Notably, gigantism is a tendency rather than a rule; many deep-sea groups show the opposite.